Register Allocation and VDD-Gating Algorithms for Out-of-Order Architectures Steven J. Battle and Mark Hempstead Drexel University Philadelphia, PA USA Email: [email protected], [email protected] Abstract—Register Files (RF) in modern out-of-order micro- 100 avg Int avg FP processors can account for up to 30% of total power consumed INT → → by the core. The complexity and size of the RF has increased due 80 FP to the transition from ROB-based to MIPSR10K-style physical register renaming. Because physical registers are dynamically 60 allocated, the RF is not fully occupied during every phase of the application. In this paper, we propose a comprehensive power 40 management strategy of the RF through algorithms for register allocation and register-bank power-gating that are informed by % of runtime 20 both microarchitecture details and circuit costs. We investigate algorithms to control where to place registers in the RF, when to 0 disable banks in the RF, and when to re-enable these banks. We 60 80 100 120 140 160 include detailed circuit models to estimate the cost for banking Num. Registers Occupied and power-gating the RF. We are able to save up to 50% of the leakage energy vs. a baseline monolithic RF, and save 11% more Fig. 1. Average Reg File occupancy CDF for SPEC2006 workloads. leakage energy than fine-grained VDD-gating schemes. 1 1 Index Terms—Computer architecture, Gate leakage, Registers, SRAM cells 0.8 0.8 I. INTRODUCTION 0.6 0.6 F.cactus I.astar 0.4 0.4 Out-of-order superscalar processors, historically found only F.gems I.libq in high-performance computing environments, are now used in F.milc I.go 0.2 F.pov 0.2 Imcf a diverse range of energy-constrained applications from smart- F.zeus Iomn phones to data-centers. Despite active research in processor % of register allocations F.avg I.avg 0 0 power management, a significant portion of active and static 0 10 20 30 40 50 0 10 20 30 40 50 power is consumed by processors’ register files. This occurs cycles cycles across computing domains; for example, the register file (RF) Fig. 2. “Allocate-write distance” CDF showing distance between register in the Motorola M-CORE embedded processor consumes allocation at rename (cycle 0) and register use at writeback. 16% of total core power [1]. This consumption is exacer- bated in modern high-performance out-of-order processors pool of rename registers eliminates false dependencies to sup- that have switched from ROB-based to MIPSR10K-based port more instructions in flight. However, application phases physical register-renaming. For example, the IBM POWER7 do not always exhibit high ILP, often leaving a significant RFs consume 21% of core power, while the Intel Westmere portion of the RF dormant. RFs account for 30% of core power [2], [3]. An additional Figure 1 shows a histogram of register files occupancy trend is the increasing contribution of static power to total across SPEC2006 benchmarks for a 160-entry register file microprocessor power consumption [4]. Again, the register modeled after Intel’s Sandy Bridge architecture [11]. On file is a significant factor: the IBM POWER7 RF and Intel average, only 68% of the RF is in use for INT workloads RF consume approximately 15% and 30% of core leakage and 78% for FP workloads. In addition, even registers that are respectively [2], [3]. Techniques such as VDD-gating [5], [6] “occupied” do not always contain valid state. Figure 2 shows and drowsy-modes [7], [8] have been used to address the the the distance in cycles between register allocation at the energy-cost of register files on a fine-grained manner, while register-rename stage and register-use during the writeback banked register files [9], [10] have been used to increase stage (“allocate-write” distance). A minimum of 6-cycles is performance and reduce dynamic costs. needed between allocate and writeback yielding two slacks Register files in modern out-of-order processors must be that can be exploited for energy reduction: slack in the amount large in order to support a large instruction window containing of RF resources available, and slack in timing when a register both architectural (committed) and speculative state; a bigger needs to be available after allocation. 978-1-4799-2987-0/13/$31.00 ©2013 IEEE 108 *+, "# $ !" $ " '( # "% " " - . !" ')( !" '( ! !" '( Fig. 4. Banked Register File: A partitioned RF allows for modular control of allocation, clock-gating, and VDD-gating when RF state is kept on a per-bank !" basis. Register reference counts monitor the RF and allow us to implement various allocation algorithms. % & allocation, keeping newly allocated registers clustered to one Fig. 3. Allocation algorithms. Each algorithm examines the set of available end of the RF. In (c), bank occupancy is compared to select the registers to select the next register for allocation. (a) Free-list: selects the reg first free register from the fullest bank, while in (d), the first at the head of the FIFO queue. (b) Prio: select first free reg from a bitvector representation of the RF (’0’=free, ’1’=allocated). (c) Full: select first free free register from the most-recently selected bank is allocated. reg from fullest RF bank. (d) MRA: select first free reg from most-recently- selected bank III. METHODOLOGY We evaluate our register file allocation and gating algo- We explore allocation and gating algorithms that are cou- rithms using both performance and circuit simulation. Our pled with microarchitectural information to reduce RF energy cycle-level performance simulator executes user level x86 64 usage. We present several algorithms using information such code, breaking x86 instructions into RISC-like three-register as instruction type, ROB activity, and register bank fullness micro-operations. The simulated core is modeled roughly after to make allocation and VDD-gating decisions. We study their Nehalem. It is 4-wide issue with a 23-stage pipeline, 128-entry efficacy in reducing energy compared to a monolithic register reorder buffer, 36-entry issue queue, and 96 rename registers. file using conventional free-list allocation. In section IV, we We model a single-threaded configuration with a register- review RF design and the circuit-costs of VDD-gating. In sec- file containing 160 registers, as described in Table I. Our tion V, we present our register allocation and RF-bank VDD- circuit models are built using the NCSU FabScalar memory gating algorithms, with results and analysis in section VI. generator [14] in NSCU’s 45nm FreePDK CMOS technology. We include measurements from HSPICE simulations to dy- II. REGISTER ALLOCATION namically model power with our performance simulator. Physical registers are allocated to instructions during the We compiled all SPEC2006 benchmarks and simulate the rename stage of the out-of-order pipeline. An allocation algo- benchmarks to completion on their training inputs, sampling rithm examines the set of free registers, providing one to the 10 million of every 500 million instructions with 10 million next dispatching instruction. Modern out-of-order processors instructions of cache and branch predictor warm-up. Note that implementing MIPS-style register renaming typically manage while our figures omit benchmarks due to space, all benchmark registers using a circular queue free-list to identify unallocated data is included in average INT and FP columns. registers [12]. Dispatching instructions are allocated a desti- nation register from the head of the free-list (a dequeuing or IV. REGISTER-FILE VDD-GATING COSTS “pop” operation). When an instruction commits its value to the When choosing a register file power management strategy, architected state, the overwritten register is freed (enqueued or it is important to ensure that the circuit costs of toggling “pushed”) to the tail of the free-list. registers do not consume more energy than they save. This Figure 3 illustrates how register allocation affects register section describes our circuit models and estimation of VDD- distribution: a 16-entry RF is shown with allocated registers gating overheads on the register file. VDD-gating is a cir- shaded. Four allocation schemes, (a)-(d), select a different cuit technique that can dramatically reduce leakage energy register according to their algorithm definition. (a) is conven- component by adding a PMOS gate transistor between the tional free-list allocation where the “next” register allocation VDD power-rail and the logic circuit [5], [15]. VDD-gating is is determined by the contents of the FIFO head-pointer. This a destructive operation and only empty registers or registers leads to registers being distributed across the RF as the whose contents are known to be expired may be gated. VDD- program executes. A priority encoded scheme is shown in gating requires a PMOS gate-transistor, driver, and additional (b), where register occupancy is represented by a bitvector isolation circuitry to ensure un-gated logic is unperturbed. The [13]. The first empty bit in the vector is selected for the next cost to switch these circuits must be recovered by the leakage 109 Component Value Component Ileak % Bank Size WPMOS1 TBE1 WPMOS2 TBE2 EPC2/EPC1 Registers 160x64-bit Precharge 174 uA 31.67% 1 (bit-cell-only) 0.180 μm 15 – – – Area 70114 um2 SRAM 152 uA 27.67% 4 12.5 μm 21 6.5 μm 23 0.66 Ports 6 read, 3 write Buffers 142 uA 25.95% 8 18.0 μm 23 9.0 μm 24 0.69 ERead 10.8 pJ WordLine 76 uA 13.90% 16 20.0 μm 21 10.0 μm 21 0.70 E 15.1 pJ Decoders 2.57 uA 0.47% Write TABLE III Latency 2 cycles SenseAmp 1.90 uA 0.35% OVERHEAD AND BREAK-EVEN FOR PMOS GATES TABLE I TABLE II BASELINE RF PARAMETERS RF LEAKAGE COMPONENTS 1 4 8 16 register file, described in Table I.